In an optical transmission system, there is known a technique of superimposing a control signal, such as a supervisory signal, on an optical signal used for transmission of data. For example, in a WDM transmission system, there is a case where a control signal for identifying an optical path is superimposed on each of optical signals in a WDM signal. In this case, node equipment or a receiver may recognize the transmission source of the optical signal and the route of the optical signal by detecting the control signal from the received optical signal.
In the above described optical transmission system, the data of respective channels are transmitted, for example, by a multi-level modulation scheme, such as QPSK and mQAM (where m is 16, 64 or 256). On the other hand, the control signal, such as a supervisory signal, is superimposed on the optical signal, for example, by frequency modulation.
As a related technique, an optical signal transmission system has been proposed, which can monitor the correspondence between a transmission source and a transmission destination of a signal without causing quality degradation of the main optical signal and without performing O/E conversion of the main optical signal (see, for example, Japanese Laid-Open Patent Publication No. 2004-40668).
In the case where the control signal is superimposed on the optical signal by frequency modulation, the receiver detects the control signal, for example, by converting frequency modulated components of the optical signal into amplitude components. However, in this case, when the frequency modulated components used for superimposing the control signal on the optical signal is large (than is, when the frequency variation width is large), the qualify (for example, error rate) of the data carried by the optical signal may be deteriorated. For this reason, it is preferred that the frequency modulated components used for superimposing the control signal on the optical signal be controlled to be sufficiently small so as to prevent the deterioration of data quality.
When the frequency modulated components on the optical signal is small, the amplitude components obtained from the frequency modulated components in the receiver is also small. In this case, the amplitude components obtained from the frequency modulated components of the optical signal are easily influenced by AM noise generated in the optical transmission path between the transmitter and the receiver. Therefore, the detection sensitivity of the control signal in the receiver decreases.